The continuous shrinking in dimensions of electronic devices utilized in ULSI (ultra large scale integrated) circuits in recent years has resulted in increasing the resistance of the BEOL metallization as well as increasing the capacitance of the intralayer and interlayer dielectric material. This combined effect, in turn, increases signal delay in ULSI electronic devices. In order to improve the switching performance of future ULSI circuits, low dielectric constant (k) insulators and particularly those with k significantly lower than that of silicon dioxide are needed to reduce the capacitance. Specifically, low k insulators having a k value of less than 4.0 are needed. Unless otherwise noted, all k values mentioned in the present application are measured relative to a vacuum.
Dielectric materials that have low k values have been commercially available; for instance, one such material is polytetrafluoroethylene (PTFE) with a k value of 2.0. However, these dielectric materials are generally not thermally stable when exposed to temperatures above 300° C.˜350° C. which renders them useless during integration of these dielectrics in ULSI chips which require a thermal stability of at least 400° C.
Typical prior art low k materials that have been considered for application in ULSI devices include polymers that contain silicon (Si), carbon (C) and oxygen (O), such as methylsiloxane, methylsesquioxanes, and other organic and inorganic polymers. For instances, materials described in a paper “Properties of new low dielectric constant spin-on silicon oxide based dielectrics” by N. Hacker et al., published in Mat. Res. Soc. Symp. Proc., vol. 476 (1997) p25 appear to satisfy the thermal stability requirement, even though some of these materials propagate cracks easily when reaching thicknesses needed for integration in the interconnect structure when films are prepared by a spin-on technique. Furthermore, the precursor materials are high cost and prohibitive for use in mass production. In contrast to this, most of the fabrication steps of VLSI and ULSI chips are carried out by plasma enhanced chemical or physical vapor deposition techniques.
The ability to fabricate a low-k material by a plasma enhanced chemical vapor deposition (PECVD) technique using previously installed and available processing equipment will thus simplify its integration in the manufacturing process, reduce manufacturing cost, and create less hazardous waste. U.S. Pat. Nos. 6,147,009 and 6,497,963 assigned to the common assignee of the present invention, which are incorporated herein by reference in their entirety, describe a low dielectric constant material consisting of Si, C, O and H atoms having a dielectric constant not more than 3.6 and which exhibits very low crack propagation velocities.
U.S. Pat. Nos. 6,312,793, 6,441,491 and 6,479,110 B2, assigned to the common assignee of the present invention and incorporated herein by reference in their entirety, describe a multiphase low k dielectric material that consists of a matrix composed of Si, C, O and H atoms, a phase composed mainly of C and H and having a dielectric constant of not more than 3.2.
Ultra low k films having a dielectric constant of less than 2.7 (and preferably less than 2.3) are also known in the art. A major problem with prior art ultra low k films is that when integrating such films in ULSI devices, the integrated films exhibit poor mechanical strength. Generally, ultra low k films have a much lower elastic modulus and hardness as compared with films with k values of approximately 2.7-3.
In view of the above drawbacks with prior art ultra low k films, there exists a need for developing PECVD processes that can produce ultra low k films that exhibit improved mechanical properties such as improved elastic modulus and hardness.